gpio: correct docs about return value of gpiod_get_direction

The use of the GPIOF_* flags is deprecated, so don't advertise them
here. Document the plain numbers for now until we have a better
solution.

Signed-off-by: Wolfram Sang <wsa+renesas@sang-engineering.com>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>